In hydrogen cooled generators, casings for shaft seals are constructed of upper and lower halves, each half consisting of radially inner and outer sections. The casings of concern in this invention are those at both ends of the generator, but the insulated collector end is of primary importance for reasons explained further herein. The basic cross section of the casing halves is generally `h` shaped, so that a cell or recess is formed at the radially inner diameter, opening toward the rotor, for housing radially inwardly projecting seal rings which, in turn, engage the rotor. Typically, each casing half, complete with a pair of 180.degree. seal ring segments held in place by garter springs and hook fits, is assembled to the generator end shield prior to testing. Once in place, testing of the casing's insulation may be done before or after the seal ring segments in the upper and lower halves are bolted together, but always after the seal ring segments are located within the casing halves, and the latter fixed to generator end shield. Should the insulation fail the test, the casing halves and seal rings must be completely disassembled, corrections made, and the casing halves and seals reassembled to the generator for retesting. The cycle is repeated as necessary, but even one disassembly/reassembly is undesirable because each time the seal rings and other delicately machined parts are exposed to possible damage. In addition, because the insulation system is redundant, the paths to ground are doubled and trouble shooting becomes much more difficult.
Another problem lies in the bolting of the seal segments in the respective upper and lower casing halves. As apparent from the discussion above, bolting of the seal ring segments in the upper and lower casing halves to each other, can be accomplished only after the casing is completely assembled to the generator. This is accomplished through small openings on the upper half of the casing which provide access to the bolts, but the procedure is problematic, and of course, whenever it becomes necessary to service the seal rings, the seal ring bolts must be removed before the casing is disassembled from the generator.
In the case of medium size generator units, the seal ring casing includes an upper half (but without a complete seal ring cell), a cap to complete the upper half, and a complete lower half The cross section of the lower half and the cap is also an `h` section so a cell is formed on the inner part of the lower casing to contain the ring segments. The upper and lower halves are assembled first to the generator, and then the insulation is tested. If the insulation fails, the nonredundant insulation system provided in these medium sized casings makes it simpler to find the problem and have it corrected. The seal ring segments per se are not assembled until the integrity of the insulation is assured. After testing the insulation successfully, the seal ring segments are installed by rolling the segments into the lower half and into the cap, where they are held in place by garter springs and hook fits on the ring segments. The casing assembly is completed by carefully sliding the cap with its seal rings into place and bolting it to the casing upper half By introducing a cap into the design, the size and weight of the part containing the segments is reduced and, by the same token, so is the possibility of damaging the rings in the assembly/disassembly process. Furthermore, the cap is the only part that needs to be disassembled for routine inspection/maintenance of the rings. Notwithstanding these advantages, however, space considerations and the cap's cross section do not allow for the access necessary for bolting the seal ring segments together in the upper and lower casing halves. As a result, it is not possible to take advantage of the higher performing bolted seal rings in the medium sized units.